1. Field of the Invention:
This invention relates to network protective devices used for controlling the connection of a primary feeder bus to a secondary network bus and more particularly to the reclosing circuitry thereof.
2. Description of the Prior Art:
A typical method by which low voltage power is furnished in locations where the load density is high and where continuity of service is to be maintained is by the use of low voltage grid or spot networks. Such networks are supplied with power from more than one set of high voltage primary feeders. Each primary feeder is connected to the network through at least one voltage reducing transformer, a network protector, and a set of fuses. If a primary feeder is lost the power formerly supplied by that feeder is supplied by the remaining feeders.
Network protector relays are designed to control the electrical connection between the network and the primary feeders. When a feeder is electrically connected to the network through a network protector the network protector relay will maintain the connection only if certain well defined conditions are met. For example, if power is flowing from the network into the feeder the feeder is to be disconnected therefrom. If a feeder is electrically disconnected from the network the network protector relay will initiate a signal for closing the network protector, thus connecting the feeder to the network, only if certain well defined conditions are met.
An example of prior art network protector relay designed to provide the above functions is disclosed in U.S. Pat. No. 3,248,609. Disconnection of a feeder from a network is determined by the direction of flow of positive sequence power. Reconnection of the feeder to the network is dependent on the proper relationship between the positive symmetrical component of polyphase voltage appearing on the feeder side of the connection and the positive symmetrical component of voltage appearing on the load side of the connection. Reconnection is also dependent on certain phase relations between the positive symmetrical component of voltage on one side of the connection and the voltage appearing across the connection itself. In a preferred embodiment reconnection of the feeder to the network is restrained by the presence of the negative symmetrical component of voltage on the load side of the connection.
Another example of a prior art network protector relay is disclosed in U.S. Pat. No. 3,947,728 which is assigned to the same assignee as the present invention. The network protector relay disclosed therein will disconnect, or trip, the feeder not only in response to the direction of the flow of positive sequence power outwardly of the network but also in response to unbalanced and simultaneous faults in the feeder network. The network protector relay disclosed therein also includes closing characeristics which will permit the automatic connection of the feeder to the network only when three conditions are met. The conditions are: that the phase sequence of the feeder voltage is the same as the phase sequence of the network voltage, that there is a proper phase relationship between the network and feeder voltages, and that the magnitude of the feeder voltage is such that power will flow into the network.